[m]u`_{y}=\frac{(x+y)`_{y}\cdot ln(z-x)-(x+y)\cdot (ln(z-x))`_{y}}{ln^2(z-x)}[/m]
[m]u`_{z}=\frac{(x+y)`_{z}\cdot ln(z-x)-(x+y)\cdot (ln(z-x))`_{z}}{ln^2(z-x)}[/m]
[m]u`_{x}=\frac{1\cdot ln(z-x)-(x+y)\cdot \frac{(z-x)`_{x}}{z-x}}{ln^2(z-x)}=\frac{ln(z-x)+\frac{x+y}{z-x}}{ln^2(z-x)}[/m]
[m]u`_{y}=\frac{1\cdot ln(z-x)-(x+y)\cdot 0}{ln^2(z-x)}=\frac{1}{ln(z-x)}[/m]
[m]u`_{z}=\frac{0\cdot ln(z-x)-(x+y)\cdot\frac{(z-x)`_{z}}{z-x}}{ln^2(z-x)}=\frac{-(x+y)\cdot\frac{1}{z-x}}{ln^2(z-x)}[/m]